Photographic printing systems have been developed which use three sequential monochrome image displays on a CRT to expose high resolution images onto photographic paper for subsequent development into a color photographic print. Typically, red, green and blue image signals are applied sequentially to the CRT to control the intensity of an electron beam which is raster scanned across a monochrome phosphor screen. A rotating color filter wheel converts the three monochrome image fields into a full color image which is projected onto the photographic paper. It is critical in a color photographic printing system of this type to produce a highly linear scan of the electron beam on the CRT screen in order to produce high resolution photographic images without artifacts introduced by a non-linear beam scan.
It is well known that flyback type of beam scan control circuits typically found in television image reproduction are limited in achieving the linearity required for photographic printing. It is also known to use controlled analog amplifier drive circuits to drive CRT deflection coils to produce highly linear beam scan. An example using digital control of deflection drive amplifiers is found in U.S. Pat. Nos. 4,142,132 and 4,687,974, in which beam position on the face of the screen is controlled according to digital position values stored in a lookup table. In this way, the beam is incrementally advanced across the face of the tube to positions determined by position values stored in the lookup table. By suitably programming these values into the table, a highly linear beam scan can be produced.
Even given this basic scan control scheme, it is difficult to produce low distortion deflection wave forms in a precision electrical scan system providing a significant amount of scan power. The difficulty is greater in higher speed scan systems. It has been found that the most difficult portion of the scan waveform to produce accurately is the region at the beginning of the scan interval immediately following the end of the retrace interval. This region brings system transient response characteristics into play, particularly those resulting from transient response characteristic of the analog amplifier circuit or circuits used to drive the CRT deflection yoke. Additionally, for reasons of production economics, the desired low distortion deflection wave form characteristics must be repeatable for all systems being produced, i.e. the solution to producing the highly linear deflection, particularly at the initial scan region, should be such as to be forgiving of reasonable tolerance variations among different CRT systems.
A well known technique of compensating for start of scan transient response nonlinearity involves modifying the scan drive waveform with an inverse curve shape of the waveform distortion. An example of this type of compensation technique applied during the entire scan interval is found in U.S. Pat. No. 5,013,978. This technique has been found, in actual application, to be inadequate for good control, i.e. elimination, of start-of-scan nonlinearity. It was found that this was due, in part, to the fact that circuit transient response at the start of scan was not identical in each machine and thus a uniform compensating waveform was not suitable for a plurality of CRT systems, even though of the same design. The alternative of developing individual compensating waveforms for each system is impractical and too costly. Moreover, a new waveform would have to be supplied any time parts of the CRT system are replaced during repairs.
It is therefore desirable to provide an improved form of linear scan control for a CRT display system which avoids the problems just discussed.